1. Field of the Invention
This invention relates to a computer system such as a personal computer, and more particularly to an improved computer system so as to access I/O devices in the system at high speeds.
2. Description of the Related Art
Recently, a wide variety of portable personal computers, such as easy-to-carry, battery-powered notebook type or laptop type personal computers, have been developed. In personal computers of this type, ISA (Industry Standard Architecture) buses are generally used.
The ISA buses were originally peripheral extension buses installed in IBM personal computers. Presently, however, they are the most standard extension buses for personal computers. The original ISA buses were used as input/output buses of 5-MHz clock and 8-bit width. Currently, however, they have been improved so as to be used as input/output buses of 8-MHz clock and 16-bit width as well.
The specifications for such ISA buses were determined conforming to the CPU (Intel 8088) employed in IBM PC/XT machines and the CPU (Intel 80286) employed in subsequent IBM PC/AT machines. Therefore, as for the CPUs down to the generation of 80286, ISA buses could be extension buses synchronized with the CPU's internal clock frequency.
For the ISA buses after that, however, to assure compatibility with many extension boards designed to be compatible with the ISA bus, the specifications for ISA buses of those days have been used up to now, without following the trend toward more powerful CPUs.
As noted above, in the past, ISA buses were extension buses synchronized with the CPU's internal clock. At present, however, as the CPU's performance is getting higher and new high-speed buses are appearing on the market, the capability of ISA buses is decreasing relatively.
Specifically, although the performance of CPUs has been getting higher from Intel 80286 to 80386, 80486, and to Pentium, . . . in that order, speeding up the CPU's internal clock and expanding the CPU bus width, the standards for ISA buses have remained unchanged, resulting in an increasing gap in performance between CPUs and ISA buses. As an substitute for ISA buses, EISA (Enhanced Industry Standard Architecture) buses, MCA (Micro Channel Architecture) buses, etc. have been devised, but they have not been widely used. ISA buses are still in use as the standard bus in the industry. One of the buses expected to be the industry standards for the next generation is the PCI (Peripheral Component Interconnect) bus. Even if PCI buses are employed, ISA buses must remain in use for some time.
Furthermore, the peripherals connected to ISA buses include I/O devices with interfaces such as IDE (Integrated Device Electronics), SCSI (Small Computer System Interface), or PCMCIA (Personal Computer Memory Card International Association). The access speeds of these I/O devices determine the performance of personal computer systems.
Consequently, the data transfer speed between the CPU and the I/O devices via a low-speed ISA bus becomes a bottleneck, which therefore limits the performance of the entire system.